Maintaining correct inflation pressure in automobile tires is accepted as an effective way to increase fuel economy, decrease tire wear and increase safety. Although proper tire inflation may be maintained by regularly checking tire pressure and adjusting accordingly, such maintenance tends to be largely ignored because of the inconvenience involved. Additionally, tire pressure varies based upon the pressurized air temperature and recent driving conditions sustained by the tire. Varying tire inflation pressure depending upon the road surface offers other benefits such as improved traction, or an increase in tire pressure may be desirable to offset changes in the vehicle payload.
The need for, and interest in, finding a practical means of maintaining proper tire pressure is illustrated by at least 19 patents granted since 1915, at least 13 of which have issued since 1979. Most of the prior art patents involve devices that are not self-contained on a wheel, but rather rely on parts mounted on the vehicle body or frame and additionally require some type of rotating joint, be it mechanical, electrical or pneumatic. Such rotating joints are expensive and impractical in the adverse environment in which vehicle wheels operate, including contaminants such as oil, water, dirt and sand which tend to attack seals necessarily a part of such rotating joints.
For example, U.S. Pat. No. 5,325,902 issued to Loewe et al., and hereby incorporated by reference, describes an automatic tire pressure monitor and inflation system having an electric motor, an air compressor disposed upon the rotating wheel, and an electrical generator. The generator includes coils mounted on the rotating wheel and a magnet mounted on a non-rotating portion of the vehicle. Thus, this system includes a rotating connection, albeit without physical contact, in which the wheel-mounted components of the generator will interact with the vehicle-mounted components thereof, to induce a voltage for driving the electric motor.
Other contemplated apparatus contained entirely on a rotating vehicle wheel include stored compressed gas or stored energy sources on the wheel which must be periodically recharged or replaced, offering limited advantages over conventional means of filling tires from stationary compressors. Thus there remains a need in the art for an apparatus fully contained on a vehicle wheel for maintaining desired inflation pressure within a tire, without the need for additional components on the frame or body portions of the vehicle, and without the need for replenishment of batteries or cartridges.